How to reduce over-fitting on training set?
The problem of overfitting is given that your model is being too flexible with the training data. The approaches you could take will be:


Download pretrained models (VGG-16) and use transfer learning.
Increase the value of your dropout (e.g. from 0.25 to 0.50).
Use data augmentation for your images.
Reduce the number of fully connected layers.